Solar thermal panels capture solar radiation in the form of heat. An absorber, typically a metal plate or foil coated with a specialized solar absorber or black paint, converts the solar radiation into heat by way of solar adsorption. The absorbed heat can be used directly for space heating, hot water generation, or solar air conditioning. Solar thermal hot water heaters typically circulate a fluid, such as water or antifreeze, in a closed loop system to transfer the accumulated heat from a solar panel to an interior of a building or fluid storage tank. Hot air solar thermal systems circulate air to transfer accumulated heat to an interior of a building or heat storage system.
A liquid fluid solar thermal system typically requires only two small holes to be drilled in mounting surface (roof, wall or other support structure) that are less than 1″ in diameter. One hole is for a cold liquid feed to a solar thermal panel and the other is for the hot liquid return from the panel. Heat can be efficiently transferred in a liquid and as a result the plumbing lines can have small diameters or cross sectional areas. In contrast, solar hot air thermal panels require large diameters or large cross-sectional area ducts to efficiently transfer heat due to the lower energy density of hot air as compared to hot water. Because the panels are installed externally onto a structure and the heat is used internally within the structure, the air solar thermal systems require two large holes, typically 6″ to 8″ in diameter cut in a roof, wall or other external support structure. One hole is for the cold air feed duct and the other hole is for the hot air return duct. Any hole cut in a roof or wall may create ingress for water resulting in leakage and other weather damage. Furthermore, the additional labor required to install the two holes versus just one adds to the cost of the solar installation. With a fluid system, the small diameter holes can be drilled and then sealed with caulk. With an airflow system, the two larger holes are preferably be drilled and cut with a saw, the mounting surface reflashed or shingled, and then the entire installation caulked for weatherproofing. In addition, with a two-hole design, the holes in both the mounting surface and the solar thermal panel is preferably precisely aligned thereby complicating the installation process.
In a solar thermal panel, the heat transfer fluid (liquid or air) should travel the maximum length of the solar absorber to absorb heat. The larger the heat transfer surface area the transfer fluid comes in contact with, the more efficient the heat transfer process becomes. In a liquid fluid system, the fluid typically travels through a serpentine pattern of tubes that are welded to the solar absorber. In an air-based solar thermal panel the air typically transverses the length of the panel. Air enters at the bottom of the panel through an inlet duct and out of the top of the panel through an outlet duct. During this process, the air picks up the heat generated by the solar absorber. The panel, however, requires a separate inlet and an outlet duct that are separated by a distance equal to the length of the panel. Alternatively, a series of air baffles can be incorporated into the panel to allow the air enter and exit at any point in the panel.
One of the major cost factors in any solar thermal panel is the cost of the solar glazing. Solar thermal panels typically employ low-iron solar glass, which permits solar radiation to strike the solar absorber but prevents heat, usually in the form of infrared radiation, from escaping. Alternatives to utilizing low-iron solar glass include transparent plastics rigid sheets, such as lexan or other polycarbonate plastic, and high-temperature optical films, such as PTFE or PVDF. However, transparent plastics often do not have the required service temperature rating and yellow with age and thin films, often only a few millimeters thick, for use in a solar thermal panel requires a method allowing automatic tensioning of the film. Without tensioning, the film will have reduced operating lifetime.